1. Field of the Invention
This invention relates to a method for calibrating a radiofrequency excitation in nuclear magnetic resonance (NMR) experimentation. The object of NMR experiments is to permit nondestructive and noninvasive determination of the intrinsic nature of bodies. Such experiments are carried out in conjunction with imaging techniques, particularly in the medical field, in order to produce images representing human-body cross-sections of patients under examination.
2. Description of the Prior Art
An NMR experiment is performed under the following conditions: a body to be examined is subjected to the influence of a steady and uniform magnetic field of high strength. The magnetic moments of the body particles then have a tendency to align themselves with the orientation of the magnetic field considered. If these particles are subjected to a radiofrequency excitation of short duration, thus inducing a signal at a so-called resonance frequency, this will permit a change in orientation of said magnetic moments. When the excitation is discontinued, these moments tend to revert to the initial orientation by precessing. The precession frequency or so-called Larmor frequency is also the resonance frequency and is related to the value of the orienting-field strength by a coefficient .gamma. known as the characteristic gyromagnetic ratio of the body. The precession signal is detected and the data relating to the body are drawn from the signal.
If the angle of slope of the magnetic moments of the particles subjected to the excitation is related to the orientation of the orienting field, it is observed that the amplitude of the response signal emitted by the body varies as the sine of said angle. If the excitation is a so-called 90.degree. excitation, the response-measured signal is in this case of maximum value. In order to obtain an enhanced signal-to-noise ratio of the response-detected signal, it is consequently useful to determine the amplitude of excitation which corresponds to this 90.degree. change in orientation of the magnetic moment of the body particles.
It may be stated in very broad terms that, in a body of given nature, the amplitude of the excitation electromagnetic field is proportional to the intensity of a so-called antenna current which passes within the radiofrequency excitation means. If said radiofrequency excitation field is designated as B.sub.1 and the antenna current is designated as I.sub.a, we may write: EQU B.sub.1 =k.I.sub.a
where k is a proportionality factor. Assuming the absence of an antenna-matching circuit, it appears that the antenna current I.sub.a is proportional to the square of the voltage V.sub.e delivered by a generator for supplying the radiofrequency excitation means, divided by the sum of the antenna resistance and the body resistance. Now it is a known fact that the body resistance with respect to the entering electromagnetic wave is approximately proportional to the fifth power of the diameter of said body. In other words, depending on whether the body to be examined is that of a patient of small stature (low resistance) or on the contrary of large stature (high resistance), calibration of the generator voltage extends over a broad range. It is known that the voltage of the generator must be calibrated between 50 and 150 volts, for example. All other things being equal, these considerations are equally valid in conventional NMR when choosing samples of different sizes.
Once a patient is in position within an NMR scanner, the first operation to be performed accordingly consists in calibrating the 90.degree. excitation in order to establish optimum conditions of examination. In examination techniques which involve the emission of excitations in accordance with the spin echo technique or of 180.degree. excitations, the problem is similar. In the case just mentioned, however, instead of calibrating the 90.degree. excitation, a known practice adopted for a number of different reasons consists in calibrating the 180.degree. excitation and in deducing the amplitude of the 90.degree. excitation therefrom by dividing by two. In all cases, the procedure consists of performing successive experiments, the amplitude of the detected signal being measured during each experiment. The generator voltage is thus caused to vary from a frst value which is, for example, lower than a given first value known as being a bottom limit for bodies of very small size to a second value which is, for example, higher than a given second value known as being necessary for excitation of bodies of large size. The curve of amplitude of the received signal is recorded as a function of the experiments and the desired value is then deduced at maximum amplitude in order to obtain a calibrated excitation.
In order to approach this maximum value with the highest possible degree of accuracy, it is necessary to vary the amplitude of excitation with a fairly small step-function interval. However, in the event of an excessively small interval of the order of 0.5 volt, for example, the calibrating operation takes too long to complete in view of the width of the range to be scanned. For example, when the operation lasts longer than twenty seconds, the user begins to suspect faulty operation of the apparatus. Speedy operation is therefore necessary. For different reasons related to the NMR phenomenon, however, successive experiments cannot be performed at a rate as high as requirements may dictate. On the contrary, they have to be separated from each other in time by an interval equal to or longer than the so-called relaxation time T.sub.1 which is characteristic of the medium to be studied. In the medical field, the relaxation time T.sub.1 has a value of the order of 500 milliseconds.
In order to shorten the calibration steps, an earlier suggestion made as an initial approach to the problem was not to scan the entire range but to stop as soon as the peak value of the reception signal has been detected and its amplitude continues to fall off in successive experiments. In other areas of investigation, one proposal consisted in resorting to the use of sub-ranges which were considered to be suited to a large number of patients while appreciably reducing the amplitude of the range. Calibration in this case is performed on the sub-ranges in a limited number of experiments such as sixteen, for example. If the maximum value has not been found in one sub-range, the entire calibration procedure is repeated, starting from one end of the range. The advantage of this type of procedure is extremely limited. In the final analysis, the number of failures experienced in this simplified method of scanning is too great to justify its use.